VolcanoesChapters 6 & 7, Hyndman & Hyndman

with input from Richard SedlockDepartment of Geology San José State University

Origin of “Volcano”• Word is from the Island of

Vulcano off Sicily

• Ancient people from the area believed that Vulcano was the chimney of the forge of Vulcan (blacksmith of the Roman gods)

• They thought hot lava fragments and clouds of dust erupting from Vulcano came from Vulcan's forge as he beat out thunderbolts for Jupiter (king of the gods) and weapons for Mars (the god of war)

http://www.volcanodiscovery.com/volcano-tours/32.html

VOLCANOES form where molten rock is vented at Earth’s surface.

Where do volcanoes form in the context of plate tectonics?Volcanoes aren’t equally dangerous....how do their hazards differ, and why?

BIG Questions

What comes out of a volcano? Ash

Fragments of solidified magmaJagged pieces of tiny rocks and glass

magnified 200 times

What comes out of a volcano? GasMost common:

H2OCO2

SO2

HCl

What comes out of a volcano?Lava

Characteristics of volcanic rocks

basalt • contains the least silica • erupts at the highest

temperature • lowest viscosity (the least

resistance to flow• basalt lava moves over

the ground easily, even down gentle slopes

dacite and rhyolite lava • tend to pile up around a

vent • form short, stubby flows

or mound-shaped domes

Illustration by J. Johnson fromhttp://volcanoes.usgs.gov/Products/Pglossary/VolRocks.html

• lower strata (layers) have hollow “well” for magma chamber

• streams of magma rise from magma chamber, through the strata

• some streams push up strata layers

• other streams move to the top opening(s) of the volcano

• raised opening that emits magma called the cone

• stream that moves to the top of the cone called the Central Vent

• volcano emits magma and a stream of tephra (ash and rock)

http://erg.usgs.gov/isb/pubs/teachers-packets/volcanoes/poster/poster.html

Mauna Loa (Hawai’i): A typical shield volcano

(also the largest volcano on earth)

Characteristics:• broad gentle slopes• formed from basalt lava of low viscosity• eruption is mostly lava rather than pyroclastic material• eruptions are not explosive (unless water gets into vents)• lava pours out of vents or as fountains

Photograph by J.D. Griggs on 25 February 1983

Mt. St. Helens: A typical composite volcano (stratovolcano)

Characteristics• steep sides, symmetrical cones• formed from magma of moderate viscosity (andesite)• alternating layers of lava flows, volcanic ash, fragments of debris• erupts explosively repeatedly from summit crater• often tall and snow covered, therefore subject to mudlfows (lahars)

Mt. St. Helens after its 1980 eruption

Explosive

Non-explosive

(“Clogged”)

(“Runny”)

Andesitic composition

Basaltic composition

Composite volcanoes

Shield volcanoes

Volcanologists recognize many types of volcanoes, but we only need to worry about two:

Volcanic HazardsLava flows

Ash fall

Pyroclastic flows

Mudflows

Volcanic Gases

Tsunami

Ash

• Composed of bits of pumice less than 2 mm across• Can drift long distances on the wind• Can stay suspended in atmosphere blocking sun and

ruining crops• Crop and roof damage - 20 cm can cause roofs to collapse• Ash eruptions generate “volcano weather” – rain• Irritates lungs and eyes - long term exposure can cause

lung disease• Reduced visibility making evacuation difficult• Planes flying into plumes of ash can have engine trouble

Mt. Pinatubo http://pubs.usgs.gov/fs/1997/fs115-97/

Mount St. Helens Ash Fallout

http://vulcan.wr.usgs.gov/Volcanoes/MSH/Maps/may18_ashmap.html

Plate-tectonic setting of volcanismExplosive (andesitic) volcanoes form at subduction zones.

Plate-tectonic setting of volcanismAt spreading centers, low pressure triggersmantle melting—fluid basaltic magma rises.

Plate-tectonic setting of volcanismWithin plates, rising plumes of hotter mantle feed

hot spots; varied volcanoes result (basaltic on Hawaii).

Evidence of Volcanoes in our own Backyard• Calistoga, Petrified Forest

• Redwood trees, 8 ft in diameter• Blown over and covered with ash from

volcano 3 million years ago• molecules of silica in the ash replaced

molecules of wood• wood turned to solid

stone

http://www.petrifiedforest.org/